1. Field of the Invention
This invention relates to the contruction and operation of gas discharge devices excited with high freqency (200 MHz to 10 GHz) electromagnetic fields of the type often used for excitation of lasers or chemical processing of solid surfaces. The invention relates to a transmission line and electrode arrangement for enabling efficient delivery of high frequency electrical power to a gas discharge plasma contained in a dielectric tube.
2. Object of the Invention
An object of the invention is to provide a compact device for efficient delivery of high frequency power from an R-F source to the discharge plasma with minimal electrical reflection due to impedance mismatch.
A second object of the invention is to provide a device which is easily tuned to compensate for changes in electrical impedance of the discharge.
A third object of the invention is to provide a plasma excitation circuit that is capable of withstanding high electrical fields.
A fourth object of the invention is to provide an electrode structure which creates a spatially uniform field in the discharge plasma.
A fifth object of the invention is to provide a means for electrically isolating low frequency electric fields which may be used for discharge initiation from the high frequency power source.
3. Description of Prior Art
Much of the prior art has emerged in the course of development of gas discharge laser and semiconductor processing technologies. Electrical circuits for excitation of gas discharge lasers by radio frequency fields have been described by Laakmann in U.S. Pat. No. 4,169,251, LaChambre, et al., Applied Physics Letters, 32(10), p. 652, May 15, 1978, and Christensen and Waynant, Applied Physics Letters 41(9), p. 794, Nov. 1, 1982, These circuits utilize lumped circuit elements for impedance matching and are not suitable for operation at frequencies in excess of approximately 200 MHz. Hollow rectangular waveguide configurations for delivery of microwave power to plasmas contained in dielectric tubes have been described by Kikuchi in U.S. Pat. No. 4,004,249 and Wynant, et al., in U.S. Pat. No. 4,413,414. These systems are extremely cumbersome and do not address the issue of matching the discharge impedance to the generator impedance for efficient power delivery. Christensen, in U.S. Pat. No. 4,631,732 has described a laser utilizing a gas discharge tube positioned between a transmission line electrode and a ground plane but again has not described external circuit elements for impedance matching or the use of specially shaped electrodes to enhance discharge uniformity.